1. Field of the Invention
The present invention relates to a switching regulator control circuit and a switching regulator using the circuit, and more particularly to a synchronous rectifier circuit in a step-up operation.
2. Description of the Related Art
FIG. 4 is a circuit diagram showing a conventional step-up/down switching regulator.
A comparator 12 compares a voltage Vd with a chopping wave to output a PWM driving signal. The voltage Vd is outputted from an error amplifier 11 having an inverting input terminal to which the voltage is inputted. The chopping wave is outputted from a chopping wave oscillator circuit 10 having a non-inverting input terminal to which the chopping wave is inputted. The output signal of the comparator 12 is supplied to a step-down switching circuit 24 through a buffer 21 as a step-down driving signal.
One non-inverting input terminal of a comparator 14 is inputted with a voltage Vu, which is obtained by shifting the output voltage Vd of the error amplifier 11 in level by Vos. Vos is set to a value equal to the amplitude voltage of the chopping wave so that the voltage Vu falls into the amplitude range of the chopping wave when the output voltage Vd of the error amplifier 11 exceeds an upper limit voltage of the chopping wave. An inverting input terminal of the comparator 14 is inputted with a chopping wave, and the chopping wave is compared with the voltage Vu to output a signal. The output signal from the comparator 14 is supplied to a step-up switching circuit 25 through a buffer 23 as a step-up driving signal.
One non-inverting input terminal of the comparator 13 is inputted with a voltage Vs, which is obtained by shifting the output voltage Vd of the error amplifier 11 in level by Vdt. An inverting input terminal of the comparator 13 is inputted with a chopping wave, and the chopping wave is compared with a voltage Vs to output a signal. The output signal from the comparator 13 is supplied to a synchronous rectification switching circuit 26 through a buffer 22 as a synchronous rectification driving signal.
Vdt is set to a value that is slightly smaller than Vos. As a result, the magnitude relation of Vs and Vu on the basis of the ground satisfies Vs>Vu, and the relation of the voltages including the chopping wave is as shown in FIG. 5. In this example, the step-up driving signal and the synchronous rectification driving signal are different from each other in duty ratio from the relation of the voltage Vs and the voltage Vu.
Accordingly, the step-up driving signal is supplied to the step-up switching circuit 25, which is turned on in response to an H level whereas the synchronous rectification driving signal is supplied to the synchronous rectification switching circuit 26, which is turned on in response to an L level, with the result that there exists a time Toff-off at which the step-up switching circuit 25 and the synchronous rectification switching circuit 26 are turned off at the same time.
The above time Toff-off prevents the output terminal and the ground from being short-circuited by turning on the step-up switching circuit 25 and the synchronous rectification switching circuit 26 at the same time. This is a technique essential to a switching regulator with synchronous rectification (for example, refer to JP 2003-180072 A).
Furthermore, in the switching regulator with the synchronous rectification, it is necessary to prevent the step-up switching circuit 25 and the synchronous rectification switching circuit 26 from always turning on when the output voltage drops and the voltage Vs and the voltage Vu are equal to or larger than the maximum Vclk(max) of the chopping wave.
In this example, each of the comparators 13 and 14 has two non-inverting input terminals, and one of those two non-inverting input terminals which inputs a lower potential is set to function whereas the other terminal which inputs a higher potential is not set to function. By using this feature, a voltage Vmax1 that is slightly lower than Vclk(max) is applied to the other non-inverting input terminals of the comparators 13 and 14. With the above structure, in the case where Vu and Vs exceed Vmax1, the comparison with the chopping wave becomes Vmax1. Accordingly, even in the case where a supply voltage is lowered, a PWM driving signal of a constant duty is outputted, which is capable of preventing the step-up switching circuit 25 and the synchronous rectification switching circuit 26 from always being turned on. Hereinafter, this state is called “maximum duty control state”.